The present invention relates to an optical disc apparatus, a focus search method, and a focus search program that achieve a quick focus search to an optical disc.
Optical discs for which a laser beam is used in recording/reproduction have been in wide use as a storage medium for audio data, video data, and other several types of digital data. Representative of the optical discs are CD (Compact Disc), DVD (Digital Versatile Disc), BD (Blu-ray Disc), etc. Representative of data-writable optical discs are CD-R, CD-RW, DVD-RW, BD-R, etc.
When an optical disc of such types is installed in an optical disc apparatus, a focus search is performed to search for an in-focus position of a data layer (in which data is recorded) of an optical disc. In a conventional focus search: a laser beam is irradiated onto an optical disc while a focal position is being continuously shifted relative to a specific position on the optical disc installed in an optical disc apparatus, to acquire a focus error signal; and then, an in-focus position of a data layer of the optical disc is determined based on a zero-cross point on an S-curve (referred to as a FES curve, hereinafter) exhibited by the acquired focus error signal.
The shift of a focal position in the conventional focus search is performed with the movement of an objective lens of an optical disc apparatus towards an optical disc under control with a focus drive voltage. The objective lens could touch the optical disc if it is moved too close to the optical disc. Therefore, it is general for the conventional focus search that: the position of the objective lens located closest to an optical disc in an initial focus search for the optical disc that is installed for the first time is set to a position relatively remote from the optical disc; and a focus search is repeated while the objective lens is gradually moved closer to the optical disc. Therefore, the conventional focus search has a problem of a long operation time
One technique to shorten the operation time of a focus search is to set a closest-position voltage that is a focus drive voltage at which an objective lens is moved closest to an optical disc, based on an in-focus position of an optical disc acquired beforehand. However, in a focus search, a peculiar in-focus position is acquired if an optical disc is inclined in an optical disc apparatus or is deformed. This is because, a FES curve is acquired by irradiating a laser beam onto a particular position on the optical disc. If a closest-position voltage is set based on a peculiar in-focus position acquired in such a condition, the closest-position voltage may be deviated from an appropriate value, thus causing a long operation time for a focus search for a next optical disc or collision of an objective lens with an optical disc.
One technique of a focus search is disclosed in Japanese Unexamined Patent Application Publication No. 01-243244. In this technique, a focus search is performed while an optical disc is rotating, to acquire a relatively average in-focus position even if the optical disc is inclined. However, in this technique, the focal position has to be shifted by a very small distance for each acquisition of an average value of a focus error signal, thus requiring a long operation time.